An electric power steering device directly uses a driving force of a motor to assist steering power of a driver of a vehicle such as an automobile. Such a vehicle to which the electric power steering device is loaded becomes widely-used. The electric power steering device makes motion of the steering wheel lighter for the driver, resulting in that it becomes lighter for the driver to operate the steering wheel without stronger power than ever.
An electric power steering device is for example represented by a pinion-assist typed electric power steering device that uses a worm shaft having a worm gear and a worm shaft as a unit that assists driving force of a motor (called “auxiliary torque”) for the purpose of the above-mentioned assistance.
As shown in FIG. 6, such a pinion-assist typed electric power steering device includes a gear-typed decelerating mechanism provided with worm wheel 101 attached to pinion shaft 100, worm shaft 103 attached to motor 102 that has worm gear 104, and so on.
Worm shaft 103 is connected to motor 102 coaxially relative to a rotational shaft of motor 102. Worm wheel 101 mating with worm shaft 103 is arranged coaxially relative to pinion shaft 100. Pinion shaft 100 is jointed to steering shaft 105. Steering shaft 105 is fixed to steering wheel 106 operated by the driver. Pinion 107 of pinion shaft 100 mates with rack-teeth 110 of rack shaft 109 for controlling wheels of the vehicle (called “wheels 108”). The auxiliary torque applied by rotation of motor 102 is, by way of the gear-typed decelerating mechanism, transmitted to rack shaft 109 by way of a mating portion of pinion 107 with rack 110.
Thus, the auxiliary torque of the motor is applied to a steering system.
However, according to such a conventional electric power steering device, a reaction against the road on which the vehicle is running, is transmitted to rack shaft 109 by way of wheel 108, resulting in vibration. The portion of motor 102 that applies the auxiliary torque to the steering system to torque transmitting unit 111 (also called “gear-typed decelerating mechanism having worm shaft 103″ attached to motor 102) becomes a generating source that causes vibration to rack shaft 109. If frequency of the vibration is consistent with frequency of the characteristic vibration of rack shaft 109, then there occurs resonance, resulting in rack shaft 109 being greatly vibrated. Further, the vibration of rack shaft 109 is transmitted from steering shaft 105 to steering wheel 106, resulting in a noise in the interior of the vehicle.
As a related art having the purpose of adjusting vibration occurring in an electric power steering device like this, e.g., there is disclosure JP11-20717A.
In this disclosure, by supporting the rack shaft at three positions in such a same way, resonance can be restricted to the vibration of the rack shaft owing to the wheels and members from the motor to the torque transmission unit. However, the supporting positions of the supporting members for supporting the rack shaft can be restricted to change to an area where the supporting positions are narrow. Thus, the resonance between the characteristic of the rack shaft and the vibration of the rack shaft caused by the members such as the wheels and the motor has not been able to be reduced, sufficiently.